It is common to express righteous anger at the extremes of inequality in poor countries. Even in the poorest countries, there is always a section of the city where shacks are replaced by houses and tended gardens. How can this small minority, the "elite" or aspirant middle class, live in such exaggerated plenty, walking past the beggars in the street, and apparently ignorant of the stark poverty of most of their fellow citizens?

But is it fair to criticise the rich in poor countries?

While it is usually discussed in economic terms, inequality is actually an issue of culture and identity. It seems to be fairly inherent in human nature to survey your peers and check how well you are doing in comparison. The key is who you think your peers are. Outsiders assume that the middle and wealthy classes in poor countries should be content with doing very well compared with those living in abject poverty, and that their relative affluence is fairly offensive as others struggle with next to nothing. But as globalisation has gathered pace, the aspirant middle classes in poorer countries do not compare themselves with the peasants or working classes in their own countries, but with the people they consider their peers in other countries – just as the elites have always done.

This is the Facebook generation. The success of Facebook across the world is the latest symbol of a profound shift in global geography: the international middle class is now a meaningful category.

A few years ago we might have called this the Friends phenomenon, after the globally successful US sitcom that had twentysomethings across the whole world watching and wishing they lived in a small New York apartment near a relaxed coffee bar. But Facebook has taken them from watching affluence on TV to regular interaction with peers in other countries. The 19 million Facebook users in the Philippines, for instance, about 19% of the population, are generally comparing themselves to other Facebook users around the globe rather than the 50% of Filipinos living on less than $2 a day.

Facebook lets the aspirant middle classes log on and see what friends or acquaintances in more wealthy countries are up to. Maybe a weekend city-hop? Or one of thousands of hobbies that the affluent engage in to pass the time. Or cooking with the latest world ingredient. Rather than being at the top of the pile, the middle classes in the poor countries, and even the "elite", actually feel like they have a lot of catching up to do on quality of life.

And government policy is made accordingly. The development model pursued in most countries is a highly unequal one, favouring wealthy groups and the aspirant middle class by promoting capital- and knowledge-intensive growth. There are other growth models that favour the poorest more – to combat inequality, economic policy needs to encourage job creation and opportunities for the poorest, including largescale investments in free or affordable basic services. But such policies are unlikely to be chosen if they prejudice the opportunities of the Facebook generation to live like their "peers" in richer countries (which they would do if they led to slower growth in wealth and wages for the already affluent).

You can read as many reports about inequality as you want (and there are lots) but the most important barrier to the introduction of pro-poor economic strategies is quite simple: lack of political will. What are the chances of the Peruvian (4 million users, 13% of the population) or Senegalese (360,000 users, 2.5% of the population) educated classes deciding to forgo the luxuries that their Facebook peers in the west (even the not very educated ones) discuss online? A car. Frequent trips abroad. The latest gadgets. Answer: about as much chance as those of us who already take all these things for granted deciding to give them up.

In the better off "middle income" countries, home to two-thirds of the world's poorest people, the Facebook generation has done well out of the last 20 years. But their fortunes contrast with the huge numbers of people still living in abject poverty in city slums or further out in the countryside. In 55% of middle-income countries inequality has increased since the 1990s, and in a further 20% it has remained high, decreasing in only a quarter of middle-income countries – according to the World Bank.

Are we witnessing the emergence of a new type of polarisation, not between countries as before, but between international income brackets? It seems that the goal of reducing inequality (and with it poverty itself) is set back when the middle-classes in poor countries set their sights on overseas living standards. But doing so is only natural – all most of them want is a better life and more opportunities for their kids.

So next time you get angry with rich Nigerians when they ignore the plight of the poor in their country, you might as well be angry with yourselves. Until we all start to share more, and rationalise our expectations in a resource-limited world, it is unrealistic to expect people in other countries to do so.